Structural effects in solvolytic reactions; carbon-13 NMR studies of carbocations: Effect of increasing electron demand on the carbon-13 NMR shifts in substituted tert-cumyl and 1-aryl-1-cyclopentyl carbocations-correlation of the data by a new set of substituent constants, sigma

Acidity constants and reactivities of the benzidine andN,N-dimethylbenzidine dications, the two electron oxidation intermediates of benzidine carcinogens

This paper describes the behavior in aqueous solutions of the two electron oxidation products of the carcinogens benzidine and N,N-dimethylbenzidine. In biological systems there is evidence that these diamines are oxidized by peroxidases, and that a product of this oxidation may be partly responsible for carcinogenicity. Entry into the oxidation products in the present study was provided through the bis-perchlorate salts of dications obtained upon chemical oxidation and through the irradiation of 4'-amino and 4'-N,N-dimethylamino-4-azidobiphenyls. The benzidine oxidation product exists in three conjugate acid-base forms, a dication, a monocation and neutral bisimine, with pKa(1) = 5.0 and pKa(2) = 9.0. These values stand in marked contrast to ones previously obtained for the two electron oxidation product of p-phenylenediamine, pKa(1) < 1.5 and pKa(2) = 5.75. The dimethylamino derivative, blocked from forming the neutral form, exists as a dication and monocation, with pKa= 5.0. Both systems are quite long-lived in aqueous solution, but they do decay on the minutes-to-hours time scale. The kinetics can be explained by reactions of both the dication and the monocation with water, with a reaction of hydroxide and the monocation becoming important around pH 10. One surprising result is that the monocations are two orders of magnitude more reactive than the dications. Thus, at neutral pH the form that exists in both systems is the monocation, and this is the species that is the most reactive towards the solvent. One of the resonance contributors in the monocation is a 4-biphenylylnitrenium ion. Comparison with other 4'-substituted-4-biphenylylnitrenium ions studied by laser flash photolysis shows that the 4'-amino- and 4'-dimethylamino substituents are highly kinetically stabilizing. These cations, for example, are a billion-fold longer-lived in aqueous solution than the parent 4-biphenylylnitrenium ion.Key words: quinone bisimine, nitrenium, aryl azide.

Carbocation-like reactivity patterns in X'-substituted-4-biphenylylnitrenium ions

4-Azido-X'-substituted biphenyls (X' = 4'-MeO, 4'-Me, 4'-F, 3'-Me, 4'-Cl, H, 3'-MeO, 3'-Cl, 4'-CF3) have been prepared and subjected to 248 nm flash photolysis irradiation in 20:80 acetonitrile:water. Transient X'-substituted 4-biphenylylnitrenium ions 10 (Ar C6H4-N + H) are observed, with lifetimes ranging from 0.6 ms (4'-MeO) to 26 ns (4'-CF3). These cations are quenched by azide ion, with values of kaz ranging from 6 to 10 x 109 M-1 s-1, with the majority in the range (9-10) x 109. This near constant kaz provides further evidence that arylnitrenium ions are quenched by azide ion at the diffusion limit. The solvent reactivities, plotted in a single-parameter Hammett plot against σ sup + (X), exhibit a poor correlation, with the points for the para π-electron donors deviating from the correlation line based on the other substituents in the direction of requiring a more negative substituent parameter. The data are more satisfactorily fit to the two-parameter Yukawa-Tsuno equation; the parameter r + obtained in this fit is 2.8. Thus the resonance interaction of the para π -donor X'-substituents with the positive charge of the cation is underestimated by σ sup + , a situation that has previously been observed with benzylic-type carbenium ions. The conclusion is made that, in their reaction with water, 4-biphenylylnitrenium ions behave like benzyl cations bearing two additional stabilizing vinyl groups, i.e., as if they had the structure Ar-C + (C = C)2. The inherent reactivity and the pattern of the aryl substituent effects are in fact similar to those in the carbocation series Ar-C + (Ph)2.Key words: nitrenium, nitrene, aryl azide, photochemistry, lifetime.

C NMR chemical shift correlations in application of "tool of increasing electron demand" to stable long-lived carbocations: Comprehensive evaluation

The reliability of (13)C NMR chemical shift correlations in the application of the "tool of increasing electron demand" to stable long-lived carbocationic systems is demonstrated by a comprehensive analysis of 22 stable aryl-substituted carbocationic systems. The observation of slopes of less than unity in such chemical shift correlations for several cationic systems has been attributed to additional charge delocalizing mechanisms present in the system (such as homoallylic, cyclopropyl, and pi conjugations). The onset of nonclassical sigma-delocalization in 2-aryl-2-norbornyl cations with electron withdrawing-substituents previously observed was further verified by using sigma(C+) substituent constants. Difficulties in relating the C(alpha)NMR shifts in different carbocationic systems are also discussed.